1. Field of the Invention
The present invention generally relates to speed indicators for vehicles and, more particularly, to an improved solid state speed indicator for railroad locomotives.
2. Description of the Prior Art
Speed indicators for railroad locomotives typically have been analog devices having a dial and a rotating indicator which points to indicia printed on the dial to provide a visual indication and measurement of speed. Current speed indicators are electrically driven, employing a meter movement. An example of this type of speed indicator is the Pulse Model SI80-XX-W Speed Indicator manufactured by Pulse Electronics of Rockville, Md. This speed indicator employs an axle driven speed sensor which generates a signal having a frequency proportional to speed. This signal is processed to produce a pulse rate signal that is supplied to the speed indicator circuitry. The speed indicator circuitry performs several functions. First, the circuitry generates a current signal proportional to speed that is used to drive the meter movement. Second, the circuitry includes overspeed detection circuits which can be preset or adjusted to a desired speed. The overspeed detection circuits include a comparator which, when an overspeed condition is detected, causes a visual indicator to be activated and deactivates a signal line connected to an external brake valve. Third, the circuitry includes calibration circuits which allows the speed indicator to be calibrated for wheel diameter.
The prior speed indicators have gained wide acceptance in the industry; however, their functions are limited and reliability is primarily determined by the ruggedness of the meter movement. It is therefore generally desirable to replace or retrofit the prior electro-mechanical speed indicators with solid state displays. By using solid state circuitry, it is also possible to provide additional functions not available on the prior speed indicators. For example, U.S. Pat. No. 4,862,433 to Bezos et al. describes a timer circuit for drag operation of trains wherein gondolas are loaded by slowly dragging a train consist past a loading chute. During such an operation, train speed is typically in the range of one third to three and one half miles per hour, a range that barely registers on the prior speed indicators.